This invention relates to voltage control circuit for protecting glow plugs which assist in starting a diesel engine, from overheating.
It is well known in the art to use glow plugs in order to improve diesel engine starting characteristic.
Heretofore, it takes about five to seven seconds to preheat a combustion chamber to a preset preheating temperature (about 900.degree. C.). In the case of a preheating speed of this order, even if the supply voltage is somewhat increased, no seriously bad influence is caused although the preheating speed may be slightly increased. However, it is rather difficult for an operator who has been familiar with gasoline engines to have a wait for a preheating time, five to seven seconds in starting the diesel engine. Accordingly, it is desirable to reduce the preheating time. In order to meet this requirement, an ultra-high-speed heating operation has this requirement, a method has been employed in which the resistance of the heat generating coil in a glow plug is greatly reduced to increase the heating current, thereby to quickly preheat the combustion chamber; i.e., a so-called "ultra-high-speed heating operation" is carried out. However, the method is disadvantageous in the following point: In the ultra-high-speed heating operation, as the resistance of the heat generating coil is extremely small, heating the glow plug response sharply and quickly to the variation of the supply voltage. Therefore, the heating speed is increased as the voltage increases. That is, overheating occurs, so that the heat generating coil is broken off or molten off.